For Asthma, Birth Order Matters
Genetic marker ups odds for first-born children, lowers them for others

(HealthDay News) -- Scientists say the long-term risk of asthma and allergies in children is affected by both the order of birth and the way a baby is delivered.

A specific genetic marker for allergic and asthmatic development that's present in a first-born child appears to increase the risk for allergic conditions for as long as 10 years after birth. But when that same genetic marker is present in a second- or third-born child, it actually lowers the risk, Dr. Wilfried Karmaus, a professor in the department of epidemiology and biostatistics at the University of South Carolina , told HealthDay .

Karmaus and his research colleagues studied more than 1,200 newborns on the Isle of Wight in Great Britain . After they recorded birth orders, they tested each newborn's allergic status by examining indicators present in umbilical cord blood. They also did standard skin prick allergy tests at ages 4 and 10 years.

The results showed that first-born children with the IL-13 gene variant were at higher risk for an allergic response and that the risk continued for the first 10 years of the children's lives. In second- and third-born children, the presence of IL-13 seemed to offer protection against allergic response.

"The fetus is, in effect, a foreign body," Karmaus said. "And a foreign body can be exposed to a lot of immune arousal or not, depending. So we think that something during pregnancy -- probably the immune system of the mother -- stimulates the IL-13 gene to act differently, depending on birth order. We haven't shown how this works yet, but that's the idea."

The study was the first to demonstrate "that birth order can affect the behavior of genes related to asthma and allergies, and that birth order can therefore affect the risk for developing one or the other," Karmaus said.

Another team of researchers found that cesarean delivery might impair regulatory cells associated with proper immune function in babies and increase the risk of asthma and allergies.

"We found a dysfunctional cellular response in the normally protective immune system among C-section babies," Dr. Ngoc Ly, an assistant professor of pediatrics at the University of California , San Francisco, told HealthDay . "And although more work needs to be done to follow how long this response might endure, we think this disrupted immune pathway may influence the development of asthma later on."

For their study, Ly and her colleagues analyzed cellular immune regulatory activity present, in the form of treg cells, in the umbilical cord blood of 50 infants born by C-section and 68 babies delivered vaginally. All babies had at least one parent with asthma or allergies.

Among the C-section babies, treg cells were more likely to operate improperly, leading to increased risk for early onset of immune system disruption, the study found. This, in turn, increases a child's risk of developing asthma or allergies.

Vaginal delivery might reduce the risk of asthma and allergies because the baby is exposed to beneficial birth canal microbes, Ly said. But she and her colleagues have to conduct much more follow-up research to see if the C-section babies do start developing asthma and allergies as they grow, she added.

Both studies were presented at an international conference of the American Thoracic Society.

On the Web

The American Lung Association has more about childhood asthma.

SOURCES: HealthDay News ; Wilfried Karmaus, M.D., professor, department of epidemiology and biostatistics, University of South Carolina, Columbia, S.C.; Ngoc Ly, M.D., assistant professor, pediatrics, University of California, San Francisco; May 21, 2008, presentations, American Thoracic Society International Conference, Toronto
Author: Robert Preidt
Publication Date: May 31, 2009
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